Edible Homestead:
November/December 2015

Understanding chill hours for deciduous fruit trees.

November 16, 2015

HomesteadIssue 15: November/December 2015


Chilling Out

In Baja Arizona, we have the privilege of being able to grow an extraordinary array of fruits and veggies at home. In addition to what we can grow in our garden beds and the citrus or fig trees that frequent our landscapes, many newcomers are surprised to learn that we can also grow peaches, apples, pears, cherries, plums, nectarines, apricots, almonds, quince, and persimmon, too.

It’s often assumed that our summers and winters are both too warm to keep deciduous fruit trees happy. It’s true that our cultivar choices seem limited when compared with cooler climates (especially in the warmest areas of Baja Arizona) but there are still a good many fruit and nut trees that really thrive here.

danny-martin_homestead-nov-dec-2015_edible-baja-arizona_03The very first step to a bountiful harvest from your own home orchard is to research the number of chill hours your area gets every year. Knowing this magic number will help you select cultivars that are genetically inclined to grow well in your climate. After you plant your trees, knowing how many chill hours you get throughout the season will give you good clues about what to expect from your tree come springtime, and what to watch and plan for over the next year.

Whenever discussing fruit trees with customers or clients, this point of the conversation brings up a lot of questions: What are chill hours, and what’s the difference between chill hours and cold hardiness? Why does a plant need a certain number of chill hours if it’s hardy enough for my area? Why does one variety need more chill hours than another? If my house gets more chill hours than a cultivar needs, then that cultivar should do fine here, right?

Chill hours can be confusing to discuss because even the experts aren’t in complete agreement about what they are. The quick and dirty definition is that they’re the cumulative number of hours that a tree spends resting during its dormant period. Calculating chill hours can be a bit befuddling, but we’ll get into that later. First, let’s look at why chill hours are so important.

What’s happening within a tree when it goes dormant?

As temperatures begin to cool and days become shorter in fall, deciduous trees prepare for dormancy by slowing production of growth-promoting hormones, and growth-inhibiting hormones begin to build up instead. Leaves change colors as chlorophyll breaks down, and their yellow, orange, or red pigments become more visible. Eventually, the growth-inhibiting hormones take over, and leaves are shed to conserve water when the tree goes dormant. In the warmest areas of Baja Arizona, it’s sometimes necessary to water sparingly in fall to help coax deciduous trees into taking a break.

Plants are much hardier to the cold once they’re resting, and shutting down many of their normal functions helps conserve energy while they protect themselves through winter. Fruit trees can tolerate extremely low temperatures while dormant, but cultivars vary in how long they like to slumber.

It takes a specific number of accumulated chill hours, longer day lengths, and warmer temperatures for a deciduous fruit tree to begin growing normally again. If one or more of these factors isn’t quite right, it will be difficult or impossible for some trees to wake up.

danny-martin_homestead-nov-dec-2015_edible-baja-arizona_01It may seem odd that a plant should need a designated amount of cold weather in order to break dormancy, but this requirement is important because it keeps the tree from resuming growth during any random winter warm spell. Such growth would certainly be damaged when normal winter temperatures returned, resulting in wasted energy, high stress, and multiple pathways for disease, fungus, or insect infestation where branch tips were damaged.

It’s best to choose cultivars that need the same number of chill hours that your area gets. If minimum chill requirements aren’t met (winter is too short) then flowering and leaf break may be prevented or delayed, quality and quantity of fruit is often reduced, and the tree may produce excessive suckers or show other signs of stress. If you get more chill hours than a tree requires, there’s a risk that it will begin growing or flowering before the final cold snaps are over.

Tracking Chill Hours

Once a tree is dormant, some scientists believe, the growth inhibiting hormones built up within the tree are slowly broken down throughout the winter, primarily when the thermometer reads between 32 and 55 degrees. However, chilling effectiveness varies with temperature. Research has found that the most effective chilling occurs between 32 and 45 degrees, and sustained warm temperatures can negate chill hours that have accumulated in the last 36 hours. Unfortunately, it seems like the more we learn about chill hours, the more confusing it gets.

The roughest method to track chill hours has us simply tally the number of hours below 45 degrees that occur while the tree is dormant. Temperatures don’t have to remain continuously below 45 degrees to count, but studies have shown that plant processes are ridiculously slow when temperatures drop below 32 degrees, so many growers count only the number of hours spent between 32 and 45 degrees. As a home grower, I can say that’s about as detailed as I’ve ever had to get with tracking chill hours. Growers in areas that experience high fluctuations in temperature are more frequently looking to a dynamic model that counts chill “portions” over a longer time frame rather than just chill hours while dormant.

Of course, the fastest and easiest way to find out about chill hours in your area is to contact the nearest county extension office or your local master gardeners. There’s a good chance that someone has already recorded and published this information for public use—and it’s also likely that these same sources can provide you with a list of specific fruit tree cultivars that do well in your area.

What to Plant

Even the very highest elevations in Baja Arizona can still plant onions in November and December, either by seed or by sets. The rest of us can also plant asparagus, beets, carrots, Swiss chard, leaf lettuce, green and bunching onions, parsley, radish, rutabaga, spinach, and turnip.

In the warmer zones, head lettuce should be planted by mid-November. Cabbage, cauliflower, collards, and endive should be planted by the first of December, and then leek and mustard by the middle of that month. Cabbage seed can be planted until mid-November if you’re below 1,000 feet elevation, and until the beginning of December above 1,000 feet. All but the very warmest zones can plant parsnip.

Garlic, kale, and kohlrabi can be still be planted this time of year, but get them in before the first of December if you live below 2,000 feet elevation. (Tucson’s airport is 2,640 feet.)

If you’re between 2,000 and 3,000 feet elevation, you can plant horseradish, rhubarb, and salsify in addition to everything in the previous paragraphs. Spring peas can be planted only in the sweet spots between 1,000 and 2,000 feet elevation until mid-December.

Broccoli and Brussels sprouts can still be planted in the warmest regions (below 2,000 feet elevation), but have them planted by Dec. 1 if you’re in the 1,000 to 2,000 feet range.

Only the lowest elevations of Baja Arizona (below 1,000 feet) will be warm enough to plant pepper and tomato (by seed), potato, and then cantaloupe, cucumber, muskmelon after Dec. 1. After around the 15th, it will likely be warm enough to plant summer squash and watermelon, too.

Amy Belk is a garden writer and photographer, a certified arborist, and a certified nursery professional who has been learning from her garden for 15 years. She and her husband homestead on a little piece of the desert in the heart of Tucson.


Winterizing Citrus

As cold weather arrives, what can you do to protect your citrus from the coming freezes?

The first step is to harden off your trees by reducing how often you water. This will slow down their growth resulting in thicker stems that are better able to cope with freezing temperatures.

When freezing temperatures do arrive, protect your trees by watering them the day before up to the evening of the freeze. The added moisture releases latent heat as the temperature drops, helping to keep the trees warm. Covering your trees with frost cloth will also aid in protecting your plants. Make sure the blankets reach the ground, if possible, and that the corners are secured to prevent the blanket from blowing away in the wind. Adding old Christmas tree lights, or any other type of heat-emitting light, will add more protection.

If the temperature is expected to be at or below 28 degrees for more than three hours, add additional frost blankets or consider leaving the water running at a slow drip overnight.

Citrus vary in cold hardiness. Lime trees are the most sensitive, withstanding temperatures only to 32; lemons are typically hardy to 30, except the Meyer lemon, which is hardy to 26. Grapefruit are hardy to 28. Mandarin oranges, better known as tangerines, are hardy down to 26, with satsuma mandarins hardy to 24. However, mandarin fruit is damaged at 28, especially if exposed for prolonged periods of time. Sweet oranges are hardy to 26, and their fruit to 27. Tangelos are hardy to between 26 to 28. Kumquats are the hardiest, all the way down to the upper teens.

Harvesting your citrus fruit when it is ripe will also help protect it from the freeze. Citrus fruit does not continue to ripen off the tree, so harvesting early will not help.

Stone fruits, such as peach, apple, pear, and plum, not only like the cold, requiring no frost protection, they also actually need a certain amount of chill hours to produce flowers.

So remember, although your citrus fruit may love a little winter chill to sweeten them up, anything more than a kiss without your protection, their heart will turn to ice.

Tony Sarah is a UA graduate in horticulture with 34 years of experience in the Tucson nursery and landscape business.

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